The present invention relates to improvements in a power assist apparatus of a steering system for an automotive vehicle, and more particularly to a steering power assist apparatus which generates a steer assisting power by supplying hydraulic oil to and discharging it from an actuator of a steering mechanism.
A variety of power assist apparatuses for a steering system have been proposed and put in practical use to improve the steering feeling for a drive and the energy consumption of a hydraulic pump. A Japanese Patent Provisional Publication No. 6-127398 discloses a power steering apparatus which includes a flow rate control valve for increasing a flow rate of hydraulic oil according to the increase of a load pressure, a first control valve of a bridge circuit which is constituted by four choke valves of semi center open type, a second control valve of a bridge circuit which is constituted by two center-close type choke valves disposed in an upstream side and two center-open type choke valves disposed in a downstream side. This conventional power steering apparatus is arranged such that each choke valve of the first control valve starts closing at the generally same time that the upstream-side choke valves start closing.
However, this conventional power steering apparatus requires to increase a base flow rate to be supplied to the control valves during a neutral condition so that this apparatus can smoothly execute steering operation in a situation that a steering operation is radically operated from the neutral condition.